Photoinduced molecular transformations. Part 132. A two-step intramolecular transposition of the 17β-acetyl group of pregnan-20-one to C-18 through the formation of cyclobutanols by the reaction of the excited carbonyl, followed by a selective β-scission of alkoxyl radicals generated from them

Abstract

New two-step transformations of a pregnan-20-one into 18-functionalized androstanes and 18a, 18b-dihomoandrostanes are described; a type-II reaction of an excited pregnan-20-one protected in the A,B-ring gave the corresponding 20-hydroxy-18,20-cyclopregnane. Selective β-scission of the cyclobutanoxyl radicals generated by irradiation of the nitrite or the hypoiodite gave a 5:4 ratio of the corresponding 18a,18b-dihomo-5α-androstan-18a-one and 18-iodoandrostan-17β-yl acetate in 89% yield or 18a,18b-dihomo-5α-androstane-17,18a-dione 17-oxime in 83% yield. The transformation involves a novel two-step intramolecular transposition of the 17β-acetyl group to C-18, and an oxygen insertion to the C-17–C-20 bond of pregnan-20-one.

Several chemoselective transformations of the functional groups of the 18-iodoandrostan-17-one and 18a,18b-dihomo-5α-pregnan-18a-one, including the synthesis of 3β-hydroxy-18a, 18b-dihomoandrost-5-ene-17, 18a-dione, are reported.